Bolometer-type detectors are used for measuring infrared, visible or ultraviolet radiation energy produced by a radiating scene. Said detectors may be arranged in an array, such as a focal plan array commonly referred to as “FPA”. Such FPA bolometer-type detector arrays are described in the documents WO 2006/100662 and WO 2006/100663, for example.
A Wheatstone bridge structure or a differential structure is also commonly used in bolometer-type detector arrays. Such a structure is represented in FIG. 1 of the document FR 2 846 666, in particular. It makes it possible to reduce the sensitivity of the measurement results supplied by the detectors, with respect to variations in the internal ambient temperature of the measurement assembly comprising the array.
Furthermore, the element of each bolometer-type detector which is sensitive to electromagnetic radiations is an electrically resistive element, in which the electric resistance varies when the detector receives radiation. This variation produces the radiation measurement. In routine practice, the radiation-sensitive resistive element is made of vanadium oxide (VOX) or amorphous silicon.
However, bolometer-type detector arrays remain highly sensitive to ambient temperature variations.
Moreover, because each detector has a different position in the array, different detectors receive different radiations, which are liable to modify the features of each detector, and the operating parameters thereof in measurement, in a manner varying between different detectors. Furthermore, initial variations exist between the detectors from the manufacture of the array, for individual detector properties such as the electric resistances and response sensitivities thereof. These initial variations also contribute to the variations between the respective responses of detectors in the same array when said array receives uniform radiation energy.
Put another way, the same radiation is liable to be measured with a different value depending on the position in the array of the detector used for the measurement thereof.
For this reason, it is known to correct deviations between bolometer-type detectors in the same array by compensating, for each detector, for the initial offset and gain variations.
To this purpose, features of each bolometer-type detector, such as the initial offset and gain thereof, are determined by means of preliminary measurements made at predefined and constant temperatures. Two lookup tables are deduced therefrom, for the gains and initial offsets of the detectors in the array, respectively. These lookup tables make it possible to compensate for the initial offset and gain of each detector and consist of corrections applied to the detection results produced by the detectors after the radiations have been detected.
However, for the measurements obtained with all the detectors of the same array to be accurate and consistent, it is necessary to update at least the initial offset correction lookup table regularly, generally every two to three minutes. This may be performed by exposing all the detectors in the array to a uniform image. Such operation is usually performed using a shutter which masks an external radiation scene uniformly with respect to the measurement assembly.
Nevertheless, the present applicant has observed that, even if the initial offset and/or gain correction lookup tables for the detectors in the array are updated frequently or regularly, this does not eliminate the persistence resulting from overexposure experienced by some bolometer-type detectors. Such overexposure may result from the exposure of at least some of the detectors of the array to an intense and/or long-term radiation flow. This flow caused long term impairment of the features of the detectors which have received it. Such overexposure may be compared to glare with a persistent effect. For example, exposing a bolometer-type detector intended to perform night measurements to the sun for a few seconds produces overexposure persistence. This persistence may last for several days, or several weeks, and may be explained by an impairment of the material of the resistive element of the bolometer which is sensitive to radiations. Such impairment is particularly probable as this sensitive material is generally in an unstable state of equilibrium, with respect to the actual physicochemical condition thereof. The impairment of the sensitive material of some of the detectors of the array produces offsets of subsequent measurement results. These offsets appear as a “phantom” image of the overexposure, superimposed on the images resulting from subsequent exposures.
Therefore, it is of particular interest to propose a method for readjusting the state of bolometer-type detectors that have been subjected to radiation overexposure, so as to restore response uniformity of all the detectors in an array.